The invention relates to chemical strengthening of glasses. More particularly, the invention relates to the use of ion exchange processes to strengthen glasses. Even more particularly, the invention relates to a process comprising multiple ion exchange processes.
Chemically strengthened glass have recently been identified for use in hand held devices, such as mobile phones, media players, and other devices, as well as other applications requiring transparency, high strength and abrasion resistance.
Ion exchange is a chemical strengthening process that starts with a glass containing smaller ions (effluent ions) that are capable of exchange with larger ions (exchange ions) in a molten salt bath at elevated temperatures—i.e., the larger ions replace the smaller ions in the glass. The larger, densely packed ions at the glass surface generate a high compressive stress, which in turn provides higher strength.
During the ion exchange process, however, the salt bath becomes increasingly diluted by the smaller effluent ions (counter ions) that are exchanged out of the glass. Salt counter ion concentration increases proportionally with the volume or number of glass parts that are ion exchanged in a salt bath. Whereas “fresh” unused salt provides the highest compressive strength, every subsequent ion exchange run increases the concentration of the smaller ions that are exchanged out of the glass and into the molten salt bath. Conversely, the concentration of the salt that provides the larger ions in the bath decreases. With continued use of the same bath, the compressive stress of the finished products is reduced. The increased dilution of the bath is typically compensated for by replacing at least a portion of the salt bath, based on ion exchange production volume or when the compressive stress of the ion exchanged glass reaches a minimum acceptable value. While such practices are adequate to maintain a minimal compressive stress in the glass, they result in significant variability and constant cyclical fluctuation in compressive stress of chemically strengthened glasses during production.
In dual stage ion exchange (DIOX), the glass undergoes ion exchange in a first, “fresh” salt bath followed by a second ion exchange in a second salt bath that has a diluted effluent ion concentration. This method sacrifices the compressive stress of the surface of the glass. This is unacceptable in hand held device applications, because it does not provide protection from surface flaws that are introduced by contact forces that may be encountered during common use of the device.